AbstractTime lag and permeation rate in liquid/liquid dialysis in general are found different from those in pervaporation. Theoretical treatments of time lag in liquid/liquid dialysis indicate that time lag, in certain cases, may be correlated to the “relaxation time” of the membrane, defined as the time required for the membrane to relax, or as the change of the permeation flux of diffusable species from initial flux to steady state due to the long time required for the dialysis solvent to reach steady state in the membrane. The permeation of dichlorobenzene in a Teflon‐FEP membrane for both the operations of liquid/liquid dialysis and pervaporation was measured as an example. It was found that the time lag in liquid/liquid dialysis is much longer than that calculated from a diffusion constant calculated from pervaporation. Permeation rate and activation energy are also different in both opera
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